Hydrogen is a potential clean and environmentally-friendly energy carrier because, with oxygen in fuel cells to generate electricity, its only product is water. It has higher energy density on a mass basis than gasoline (120 MJ/kg for hydrogen vs. 44 MJ/kg for gasoline) but far lower volumetric energy density (0.01 MJ/L for hydrogen at STP vs. 32 MJ/L for gasoline). See Satyapal, S.; Petrovic, J.; Read, C.; Thomas, G.; Ordaz, G., The US Department of Energy's National Hydrogen Storage Project: Progress towards meeting hydrogen-powered vehicle requirements. Catalysis Today 2007, 120, 246-256. Therefore, Hydrogen powered fuel cell vehicles are expected to play a key role in future transportation systems since they produce only electricity, heat, and water at point of use. A major obstacle for the development of hydrogen powered vehicles is the lack of safe, light weight and energy efficient means for on-board hydrogen storage. See Schlapbach, L.; Zuttel, A., Hydrogen-storage materials for mobile applications. Nature 2001, 414, (6861), 353-358.
Current approaches for hydrogen storage include: compressed hydrogen gas, cryogenic and liquid hydrogen, sorbents, metal-organic frameworks (MOFs), metal hydrides and chemical hydrides. See Chen P, Xiong Z T, Luo J Z, Lin J Y, Tan K L. Interaction of Hydrogen with Metal Nitrides and Imides. Nature 2002; 420: 302-4; Satyapal S, Petrovic J, Read C, Thomas G, Ordaz G. The US Department of Energy's National Hydrogen Storage Project: Progress Towards Meeting Hydrogen-powered Vehicle Requirements. Catal Today 2007; 120: 246-56; Maus, S.; Hapke, J.; Ranong, C. N.; Wuchner, E.; Friedlmeier, G.; Wenger, D., Filling procedure for vehicles with compressed hydrogen tanks International Journal of Hydrogen Energy 2008, 33, (17), 4612-4621; Ahluwalia, R. K.; Peng, J. K., Dynamics of cryogenic hydrogen storage in insulated pressure vessels for automotive applications. International Journal of Hydrogen Energy 2008, 33, (17), 4622-4633; Juan-Juan, J.; Marco-Lozar, J. P.; Suarez-Garcia, F.; Cazorla-Amoros, D.; Linares-Solano, A., A comparison of hydrogen storage in activated carbons and a metal-organic framework (MOF-5). Carbon 2010, 48, (10), 2906-2909; Bluhm, M. E.; Bradley, M. G.; Butterick, R.; Kusari, U.; Sneddon, L. G., Amineborane-based chemical hydrogen storage: Enhanced ammonia borane dehydrogenation in ionic liquids. J Am Chem Soc 2006, 128, (24), 7748-7749; Marrero-Alfonso, E. Y.; Beaird, A. M.; Davis, T. A.; Matthews, M. A., Hydrogen Generation from Chemical Hydrides. Industrial & Engineering Chemistry Research 2009, 48, (8), 3703-3712; Mori D, Hirose K. Recent Challenges of Hydrogen Storage Technologies for Fuel Cell Vehicles. International Journal of Hydrogen Energy 2009; 34: 4569-74; and Schuth F. Challenges in Hydrogen Storage. Eur Phys J-Spec Top 2009; 176: 155-66.
Pressurized tanks and cryogenic liquid hydrogen provide viable means for stationary hydrogen storage, but challenges remain in their use for on-board vehicles. Their main disadvantages are the large physical volume required, and the energy penalty associated with compressing the gas to high pressures (typically 5,000-10,000 psi). There are also issues that have not been fully resolved, such as the problem of rapid loss of H2 in an accident. Recently, a wide range of nanoporous materials has been studied as potential hydrogen storage media. The advantage of sorbent materials lies in their ready reversibility. Due to the weak interaction between molecular hydrogen and sorbent, however, this approach requires low temperatures (normally about 77° K) to obtain reasonable hydrogen uptake, which is difficult to maintain in a vehicle application.
Metal hydrides and chemical hydrides are the most promising approaches for hydrogen storage due to the high gravimetric capacity and moderate storage/release temperature. Metal and complex hydrides rely on stronger chemical interactions than sorbents, and thus can store hydrogen at higher temperatures. This approach provides superior hydrogen capacity 8-11 wt %. However, high temperatures (>300° C.) are required to liberate hydrogen, but this temperature level is not available in proton exchange membrane fuel cells (“PEM FCs”), which operate at about 85° C. Chemical hydrides offer the advantages of high hydrogen gravimetric capacity, along with ease of hydrogen release. Unlike reversible metal or complex hydrides, however, dehydrogenation process of chemical hydrides is not reversible. Thus, the spent material must be removed from the vehicle for off-board regeneration.
Among chemical hydrides, ammonia borane (NH3BH3, “AB”) has attracted considerable interest as a promising hydrogen storage candidate because of its high hydrogen content (19.6 wt %), hydrogen release under moderate conditions, and stability at room temperature. See Stephens, F. H.; Pons, V.; Baker, R. T., Ammonia-borane: the hydrogen source par excellence? Dalton Transactions 2007, (25), 2613-2626; Wang P, Kang X D. Hydrogen-rich Boron-containing Materials for Hydrogen Storage. Dalton T 2008: 5400-13; and Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I., B—N compounds for chemical hydrogen storage. Chem Soc Rev 2009, 38, (1), 279-293.
TABLE 1Hydrolysis and Thermolysis EquationsHydrolysis:NH3BH3 + 3H2O → B(OH)3 + NH3 + 3H2(1) Thermolysis:                    NH        3            ⁢              BH        3              →                            1          x                ⁢                              (                                          NH                2                            ⁢                              BH                2                                      )                    x                    +              H        2              ;      (          90      ⁢              -            ⁢      120      ⁢      °      ⁢                          ⁢              C        .              )  (2)                     1        x            ⁢                        (                                    NH              2                        ⁢                          BH              2                                )                x              →                            1          x                ⁢                              (            NHBH            )                    x                    +              H        2              ;      (          150      ⁢              -            ⁢      170      ⁢      °      ⁢                          ⁢              C        .              )  (3)                     1        x            ⁢                        (          NHBH          )                x              →                            1          x                ⁢                              (            NBH            )                    x                    +              0.5        ⁢                  H          2                      ;      (          >              150        ⁢        °        ⁢                                  ⁢                  C          .                      )  (4)                     1        x            ⁢                        (          NHBH          )                x              →                            1          x                ⁢                              (            NB            )                    x                    +              H        2              ;      (          >              500        ⁢        °        ⁢                                  ⁢                  C          .                      )  (5)
There are two distinct approaches for AB dehydrogenation: (1) hydrolysis using catalysts (Table 1, Eq. 1), which generates borates and ammonia, and (2) thermolysis (Table 1, Eq. 2-5), which generates various products such as (poly)aminoborane, (poly)iminoborane, cyclotriborazane, borazine (N3B3H6), polyborazylene, etc. See Yan, J. M.; Zhang, X. B.; Akita, T.; Haruta, M.; Xu, Q., One-Step Seeding Growth of Magnetically Recyclable Au at Co Core-Shell Nanoparticles: Highly Efficient Catalyst for Hydrolytic Dehydrogenation of Ammonia Borane. J Am Chem Soc 2010, 132, (15), 5326-5327; Jiang, H. L.; Umegaki, T.; Akita, T.; Zhang, X. B.; Haruta, M.; Xu, Q., Bimetallic Au—Ni Nanoparticles Embedded in SiO2 Nanospheres: Synergetic Catalysis in Hydrolytic Dehydrogenation of Ammonia Borane. Chem-Eur J 2010, 16, (10), 3132-3137; Ramachandran, P. V.; Gagare, P. D., Preparation of ammonia borane in high yield and purity, methanolysis, and regeneration. Inorg Chem 2007, 46, (19), 7810-7817; Nylen, J.; Sato, T.; Soignard, E.; Yarger, J. L.; Stoyanov, E.; Haussermann, U., Thermal decomposition of ammonia borane at high pressures. J Chem Phys 2009, 131, (10), 104506 (1-7); Baitalow, F.; Wolf, G.; Grolier, J. P. E.; Dan, F.; Randzio, S. L., Thermal decomposition of ammonia-borane under pressures up to 600 bar. Thermochim Acta 2006, 445, (2), 121-125; Baitalow, F.; Baumann, J.; Wolf, G.; Jaenicke-Rossler, K.; Leitner, G., Thermal decomposition of B—N—H compounds investigated by using combined thermoanalytical methods. Thermochim Acta 2002, 391, (1-2), 159-168; Heldebrant, D. J.; Karkamkar, A.; Hess, N. J.; Bowden, M.; Rassat, S.; Zheng, F.; Rappe, K.; Autrey, T., The Effects of Chemical Additives on the Induction Phase in Solid-State Thermal Decomposition of Ammonia Borane. Chem Mater 2008, 20, (16), 5332-5336.
Hydrolysis provides low theoretical H2 yield due to limited AB solubility in water and requires catalysts. See Metin O, Mazumder V, Ozkar S, Sun S S. Monodisperse Nickel Nanoparticles; and Their Catalysis in Hydrolytic Dehydrogenation of Ammonia Borane. J Am Chem Soc 2010; 132: 1468-9; and Wang P, Kang X D. Hydrogen-rich boron-containing materials for hydrogen storage. Dalton T 2008: 5400-13.
In addition, AB generates B—O bonds which are not preferred from the spent fuel regeneration viewpoint, and NH3 which must be removed for use in proton exchange membrane fuel cells (“PEM FCs”). See Smythe N C, Gordon J C. Ammonia Borane as a Hydrogen Carrier: Dehydrogenation and Regeneration. Eur J Inorg Chem 2010: 509-21; Uribe F A, Gottesfeld S, Zawodzinski T A. Effect of Ammonia as Potential Fuel Impurity on Proton Exchange Membrane Fuel Cell Performance. J Electrochem Soc 2002; 149: A293-A6; Hwang H T, Al-Kukhun A, Varma A. Hydrogen for Vehicle Applications from Hydrothermolysis of Ammonia Borane: Hydrogen Yield, Thermal Characteristics, and Ammonia Formation. Industrial & Engineering Chemistry Research 2010; 49: 10994-1000; and Al-Kukhun A, Hwang H T, Varma A. A Comparison of Ammonia Borane Dehydrogenation Methods for Proton-Exchange-Membrane Fuel Cell Vehicles: Hydrogen Yield and Ammonia Formation and Its Removal. Ind. Eng. Chem. Res.: 10.1021/ie102157v.
On the other hand, thermolysis requires either relatively high temperature (>150° C.) to release 2 or 2.5 equivalents of hydrogen per AB, or relatively costly additives (which constitute weight penalty) for lower temperature operation and shorter induction period. See Baitalow, F.; Baumann, J.; Wolf, G.; Jaenicke-Rossler, K.; Leitner, G., Thermal decomposition of B—N—H compounds investigated by using combined thermoanalytical methods. Thermochim Acta 2002, 391, (1-2), 159-168; Heldebrant, D. J.; Karkamkar, A.; Hess, N. J.; Bowden, M.; Rassat, S.; Zheng, F.; Rappe, K.; Autrey, T., The Effects of Chemical Additives on the Induction Phase in Solid-State Thermal Decomposition of Ammonia Borane. Chem Mater 2008, 20, (16), 5332-5336; Neiner, D.; Karkamkar, A.; Linehan, J. C.; Arey, B.; Autrey, T.; Kauzlarich, S. M., Promotion of Hydrogen Release from Ammonia Borane with Mechanically Activated Hexagonal Boron Nitride. J Phys Chem C 2009, 113, (3), 1098-1103; Gutowska, A.; Li, L. Y.; Shin, Y. S.; Wang, C. M. M.; Li, X. H. S.; Linehan, J. C.; Smith, R. S.; Kay, B. D.; Schmid, B.; Shaw, W.; Gutowski, M.; Autrey, T., Nanoscaffold Mediates Hydrogen Release and the Reactivity of Ammonia Borane. Angewandte Chemie-International Edition 2005, 44, (23), 3578-3582; and Hu, M. G.; Geanangel, R. A.; Wendlandt, W. W., Thermal-Decomposition of Ammonia-Borane. Thermochim Acta 1978, 23, (2), 249-255. Above 500° C., complete dehydrogenation occurs forming boron nitride (BN). See Baitalow, F.; Baumann, J.; Wolf, G.; Jaenicke-Rossler, K.; Leitner, G., Thermal decomposition of B—N—H compounds investigated by using combined thermoanalytical methods. Thermochim Acta 2002, 391, (1-2), 159-168; Himmelberger D W, Alden L R, Bluhm M E, Sneddon L G. Ammonia Borane Hydrogen Release in Ionic Liquids. Inorg Chem 2009; 48: 9883-9; Himmelberger D W, Yoon C W, Bluhm M E, Carroll P J, Sneddon L G. Base-Promoted Ammonia Borane Hydrogen-Release. J Am Chem Soc 2009; 131: 14101-10; and Hu, M. G.; Geanangel, R. A.; Wendlandt, W. W., Thermal-Decomposition of Ammonia-Borane. Thermochim Acta 1978, 23, (2), 249-255. From a spent fuel regeneration viewpoint, however, BN is not preferred due to its high chemical and thermal stability. See Smythe, N. C.; Gordon, J. C., Ammonia Borane as a Hydrogen Carrier: Dehydrogenation and Regeneration. Eur J Inorg Chem 2010, (4), 509-521.
It has also been reported that even neat AB thermolysis generates some ammonia. See Hwang H T, Al-Kukhun A, Varma A. Hydrogen for Vehicle Applications from Hydrothermolysis of Ammonia Borane: Hydrogen Yield, Thermal Characteristics, and Ammonia Formation. Industrial & Engineering Chemistry Research 2010; 49: 10994-1000; Al-Kukhun A, Hwang H T, Varma A. A Comparison of Ammonia Borane Dehydrogenation Methods for Proton-Exchange-Membrane Fuel Cell Vehicles: Hydrogen Yield and Ammonia Formation and Its Removal. Ind. Eng. Chem. Res.: 10.1021/ie102157v; and Neiner D, Karkamkar A, Linehan J C, Arey B, Autrey T, Kauzlarich S M. Promotion of Hydrogen Release from Ammonia Borane with Mechanically Activated Hexagonal Boron Nitride. J Phys Chem C 2009; 113: 1098-103. Above 500° C., AB can be completely decomposed to form boron nitride (BN). For spent fuel regeneration, however, BN is not preferred due to its high chemical and thermal stability. See Smythe N C, Gordon J C. Ammonia Borane as a Hydrogen Carrier: Dehydrogenation and Regeneration. Eur J Inorg Chem 2010: 509-21.
For use in PEM FCs, ammonia and borazine (a cyclic volatile compound) need to be removed from the H2 stream. It has been reported that as low as 13 ppm NH3 can decrease the fuel cell performance, and that the degradation is irreversible for long term exposure (15 hr) to 30 ppm NH3. See Uribe, F. A.; Gottesfeld, S.; Zawodzinski, T. A., Effect of ammonia as potential fuel impurity on proton exchange membrane fuel cell performance. Journal of the Electrochemical Society 2002, 149, (3), A293-A296. The U.S. Department of Energy (DOE) has set the target for ammonia concentration at <0.1 ppm in hydrogen for PEM FC. See SAE-J2719, Information Report on the Development of a Hydrogen Quality Guideline for Fuel Cell Vehicles. Society of Automotive Engineers 2008. While ammonia even in minute amounts is intolerable, ammonia is produced as a byproduct with most dehydrogenation processes.
Therefore, there is a need for an improved system and method which enables high liberation of hydrogen while minimizing ammonia formation. Additionally, an efficient process is needed to remove ammonia from a hydrogen carrier composition during hydrogen generation processes. In particular, a method and process to improve purity of the hydrogen stream entering the fuel cell is needed.